The Macadamia Tree

Educational information for schools

In the beginning

When the continents of the earth were forming and South America, Africa, India, the Middle East and Australia were loosely joined, a tree evolved as a common ancestor of the family now known as the Proteaceae. The landmass separated into the form we know today and the Proteaceae developed into about 75 families or genera. This occurred by some combination of natural selection, hereditary variation and evolution.

About 50,000,000 years ago one variation existed in a form we would today recognize as the genus Macadamia.

In Australia, the layman will see many trees which have a similarity to the Macadamia and it is easy to understand the difficulties in identification, which took almost 100 years to resolve.

The genus Macadamia consists of two distinct, but allied groups divided into tropical and subtropical types. The tropical groups are native only to northeast Australia and the Celebes Island and according to current knowledge, consist of the species Macadamia grandus and Macadamia whelani. These are both big trees producing large, inedible fruit.

Taxonomy

The macadamia tree belongs to the Proteaceae family, series Folliculares and the tribe Grevillea which includes Australian natives Buckinghamia, Grevillea and Hakea. In the same family but belonging to other genera are Banksia, Conospermum, Dryandra, Isopogon, Lomatia, Persconia and Stanocarpus.

Grandus, Jansenii, Ternifolia, Integrifolia, Tetraphylla, Whelani.

There are only three other edible nuts that belong to the Proteaceae family – the gevuina nuts of Chile Gevuina avellana, the helica nut Helica diversifolia and the rose nut Hicksbeachia pinnatifolia.

To date many species of macadamia have been identified, but only two are known to produce edible nuts – M.integrifolia and M.tetraphylla.

Botany

Macadamia integrifolia dominates over Macadamia tetraphylla in the growng of macadamia nuts. The reasons for this are: the higher sugar content of M.tetraphylla, which leads to browning of the kernels when roasted; M.integrifolia is more resistant to water stress; research, selection work and breeding programs have mainly focused on integrifolia.

Both species are native to the east coast of Australia, ranging from 25° to 31° south. Roughly from Bundaberg to Coffs Harbour. It is believed that tetraphylla is native to the southern half of this range and integrifolia to the northern half. Integrifolia performs better in warmer conditions. Southern growers use tetraphylla as root stock for grafting integrifolia.

These trees are beautiful, dark green, heavily foliated evergreens. Distinguishing features are shown in the leaves where m.integrifolia has three leaves per whorl and tetraphylla has four. Integrifolia has a less spikey leaf than its sister. Flowers on the former are mostly white whereas the latter are often pink.

The nut or seed surfaces are easily identified as integrifolia is generally smooth and the tetraphylla is rough or pebbled.

Macadamia Integrifolia

Integrifolia is roundish in shape and a mature tree can attain a height and spread of 20 metres with an average height of 12 – 15 metres.

The average trunk diameter is around 300mm. The root is of the dicot type producing a tap upon germination followed by growth of lateral roots.

The leaf blade is 75 – 225mm in length with some up to 300mm. New flushes are pale green, often with a light violet tip.
Flowers have a pleasant, sweet smell and are borne on long sprays called racemes which hang from the axils of leaves. The mature racemes vary from 100mm to 300mm in length and carry 100 to 300 flowers. About 10% of these will eventually form ‘nutlets’ and ripen into nuts.
The mature fruit measures roughly 25mm in diameter. The outer bright green pericarp is about 3mm thick and conceals a brown seed or nut that consists of an outer, hard shell or testa, 2 to 5mm thick, and an inner cream-coloured kernel.

It takes around 185 days from fruit set to maturity. The weaker baby fruit drop continuously until 50 to 60 days after fruit set. In its natural state a macadamia tree will have flowers, nutlets and mature nuts growing simultaneously, in profusion for much of the year. The nuts fall to the ground between March and September.
Tree numbers actually declined during the 1940s and 1950s. Native trees suffered from forest clearing and orchards did not produce well due to seedling variability, insect pests and lack of knowledge. The inability to crack them was a major restriction and this resulted in lower than expected returns. Many orchards became overgrown by native vegetation. From a total planting of about 900 acres over sixty years, only about 100 remained.